1. Field of the Invention
This invention relates to improvements in a machine for making spiral pipe from a strip of sheet metal. More particularly it concerns means and methods for accurately controlling the diameter of such pipe. As used herein the terms "tubing", "pipe", and "conduit" are used interchangeably and include all diameters and wall thicknesses or gauges, both corrugated and non-corrugated and lock seam or welded seam.
2. Description of the Prior Art
The improvement of this invention pertains to a machine of the type described in U.S. Pat. No. 3,247,692. In such a machine an elongated strip of sheet metal is driven at an oblique angle through three rolls, a guide roll, an anvil roll and a pressure roll, offset from each other to bend the strip, using the anvil roll as a fulcrum, into convolutions which, because of the entry angle of the strip, are helical. The resulting helical convolutions are joined at adjacent edges in a continuous seam by various means, such as a pair of lock up rolls positioned on opposite sides of the strip in line with the anvil roll which crimp preformed flanges on opposed longitudinal edges of the strip together in an interlocking relationship.
Three basic factors affect the diameter of the tubing made by such a three-roll machine: the width of the strip, the angle of entry of the strip through the rolls and the position of the pressure roll relative to the other two rolls. In operation, these three factors are held constant in an attempt to maintain the diameter of the tubing constant. However, due to various discrepancies resulting from the forces exerted on the three-roll structure by the weight, gauge (thickness) and diameter of the pipe and inherent variations in the strip camber, gauge, temper and the like, the strip may not be bent uniformly and the diameter of the finished pipe may "wander". Obviously, where uniform diameter is important, such as in the manufacture of pipe sections which are to be joined to other sections at their point of use or in the manufacture of conduit for heating ducts to be extended through preformed wall apertures, significant variation in the tubing diameter cannot be tolerated. Accordingly, inasmuch as the machine operator has no control over the strip width, adjustments of either or both of the entry angle and relative position of the pressure roll were required in order to control the diameter within specifications. Normally, such adjustments could only be made by temporarily shutting down the machine.
It has also been suggested that diameter control might be accomplished by varying the "lap" (depth of interlocking) of the flanges on the strip edges before crimping them together into a lock seam. In both welded-seam pipe and lock-seam pipe the relationship of the edges of the strip at the point of joining is exceedingly important. In order to form a proper welded seam the edges should abut with predetermined pressure and a properly formed lock seam is "full" in that the flanges are completely overlapped. Varying the lap of the edges by moving one or both of the lock up rolls axially relative to the tubing axis results in a nonuniform seam whose strength varies along its length depending upon the degree of lap effected. In bendable conduit, especially, such nonuniformity is undesirable and may result in buckling or seam separation.